bnx2x: Keep only one HW path active

Disable bmac access while working with emac and keep the single lane SerDes in
reset while working with 4 lanes XGXS

Signed-off-by: Yaniv Rosner <yanivr@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>